An X-ray CT (Computed Tomography) apparatus is an apparatus which images the interior of a subject by scanning the subject with X-rays, acquiring detection data, and reconstructing the acquired detection data with a computer.
The X-ray CT apparatus comprises a rotator at which an X-ray tube and an X-ray detector are arranged opposite to each other on a circular frame, wherein the rotator is structured such that a subject is positioned inside the frame, irradiated with X-rays from the X-ray tube during rotation of the rotator, and scanned by detecting the X-rays transmitted through the subject with the X-ray detector.
Imaging methods using the X-ray CT apparatus include an imaging method known as step and shoot. In this method, after irradiating a part of a subject with X-rays to create an image, the subject with a couch is moved a prescribed distance in the rostrocaudal direction while stopping the irradiation of X-rays, another part of the subject is then irradiated with X-rays to create an image, and by repeating the operations of moving and imaging, and composing the images of each part of the subject, an image of a broader range of the subject is obtained. In general, the images of each part of the subject are imaged so as to include superposed regions (overlap regions) to be used for the composition.
Another imaging method using the X-ray CT apparatus is an imaging method known as synchronous (gated) imaging. In this method, biological signals of the subject are received and X-rays are irradiated at prescribed phases of these signals. Known examples of the synchronous imaging include ECG (electrocardiogram)-gated examination and respiratory-gated imaging.
The ECG-gated examination is a method in which electrocardiographic signals are received from an electrocardiograph as biological signals of the subject, and X-rays are irradiated at prescribed cardiac phases of these signals. With this method, in some cases, the remaining range in the prescribed cardiac phases may be set in advance to perform half imaging based on this remaining range.
The respiratory-gated imaging is a method in which respiratory signals are received from a respiratory measuring unit as biological signals of the subject, and X-rays are irradiated at prescribed respiratory phases of these signals. With this method, in some cases, the remaining range in the prescribed respiratory phases may be set in advance to perform the half imaging based on this remaining range.
Moreover, methods of reconstruction process used for a cone-beam X-ray CT apparatus which irradiates X-rays in a conical form include a method known as FDK (Feldkamp, Davis, Kress) reconstruction technique, as well as a method, in which the FDK reconstruction technique is expanded, known as image region expansion reconstruction technique. Images reconstructed using the FDK reconstruction technique are three-dimensional images in which the cross-sections parallel to the rostrocaudal direction of the subject (the sagittal cross-section and the coronal cross-section) form a hexagon. In comparison, images reconstructed using the image region expansion reconstruction technique are three-dimensional images in which the image region has been expanded so that the cross-sections parallel to the rostrocaudal direction of the subject form a rectangle.
In this way, the images created using the image region expansion reconstruction technique have the region broader than that of the images created using the FDK reconstruction technique. Consequently, with imaging using the image region expansion reconstruction technique, it is possible to obtain images over a broader range of the subject with fewer rounds of X-ray irradiation compared to imaging using the FDK reconstruction technique. Furthermore, due to the abovementioned variance in the shapes of the cross-sections parallel to the rostrocaudal direction of the subject, with imaging using the image region expansion reconstruction technique, it is possible to make the superposed regions of the images smaller compared to imaging using the FDK reconstruction technique. As a result, imaging using the image region expansion reconstruction technique allows for imaging with lower radiation exposure compared to imaging using the FDK reconstruction technique. However, it is known that the image region expansion reconstruction technique requires an amount of detection data that is at least equivalent to a full scan. In other words, the image region expansion reconstruction technique requires detection data that is the amount of detection data represented by a number of rotations of the rotator is 1 or more. Consequently, the feasibility of using image region expansion reconstruction technique depends on the amount of detection data. The amount of detection data is determined based on the imaging conditions (the rotational speed of the rotator, the abovementioned remaining range, and the like).